The quick answer to this is that the author shows that the sea is calm by saying that it is "as smooth as a plate glass window."

This is a simile because the author is trying to describe how smooth the sea is by comparing it to the window. Importantly, he says that it is "as smooth as" the window. When you explicitly compare two things using the word "like" or "as," it is a simile.

It would have been a metaphor if the author would have said "the sea was a plate glass window."